Corrosion-inhibitive heat transfer compositions of aqueous alcohol solutions heretofore have been widely employed in heat exchange systems associated with internal combustion engines, solar systems, and the like, to depress the freezing point of the coolant, to raise its boiling point, and provide corrosion inhibition. Since conventional alcohol-based heat transfer fluids, such as automobile antifreezes, have the tendency to corrode the metal (e.g., aluminum, copper, iron, brass and solder) surfaces of the heat transfer system, these fluids generally contain corrosion inhibitors to minimize this phenomenon.
Two United States patents are particularly noteworthy, as these are considered to be representative of the state-of-the-art antifreeze technology. U.S. Pat. Nos. 3,341,469 and 3,337,496 thus disclose an aqueous alcohol composition employing organosiloxane/silicate copolymers wherein the siloxane can contain polyethyleneoxy organosilicon species. As an additional illustration, U.S. Pat. No. 3,198,820 discloses an aqueous alcohol antifreeze containing a carboxyl organosiloxane/silicate copolymer.
Corrosion-inhibitive heat transfer compositions of the type described in the '469 and '496 patents have enjoyed wide usage. A composition of this type typically has superior shelf-life, does not attack rubber parts in cooling systems, is characterized by low levels of foam formation, and is useful over a wide temperature range even after prolonged periods in service.
In addition to providing corrosion protection for cooling system metals, the art has recognized the need to provide an antifreeze which does not gel in concentrate form. Thus, U.S. Pat. No. 4,149,985 teaches that the pH at the time of silicate addition to such antifreeze concentrates must be between 9.5 and 10 5 (providing a final concentrate pH of about 11) in order to minimize concentrate silicate stability problems during storage. Unfortunately, these antifreeze concentrates are subject to annoying gelation problems from time to time, although these antifreezes afford reasonable latitude in preparation and use.
Many prior patents in this field, e.g., the '469, '496, '985 and '820 patents identified herein, suggest that an extremely large number of silanes are useful to form stable polymers with inorganic silicates which, in ethylene glycol concentrates, are not subject to gelation and the manufacturing and dispensing problems associated with gel formation. These prior patents likewise suggest that the pH range may vary widely, typical ranges disclosed being from 7.0 up to 12.0 or so.
Prior work in this field also suggests that relatively large amounts of silicates (as much as 5,000 ppm of Si) can be appropriately stabilized. However, it has been found that these prior art suggestions are not particularly useful in predicting the suitability of silanes for generating effective silicate species in aqueous antifreezes.
Pursuant to the invention disclosed in commonly-assigned U.S. application Ser. No. 752,561, it has been found that relatively low pH antifreezes of less than about 10, having concentrate pHs of between about 5.8 and about 7.5, impart highly effective protection against aluminum corrosion in comparison to state-of-the-art antifreezes and are silicate gel resistant in the concentrate when using selected silanes.
Still other prior work in this field is reflected in British Pat. No. 2,018,266A and U.S. Pat. Nos. 4,333,843, and 4,386,154. In general, this prior work suggests the use of a wide variety of siliconate/phosphonate compounds as stabilizers against gel formation of silicates in an antifreeze. The use of the alkali metal and tetraalkyl ammonium derivatives is also disclosed.
In addition to the foregoing, the prior art is replete with attempts to provide antifreeze formulations which are gel resistant and impart the desired protection for engine cooling systems. The essentially uniform focus, applicants believe, has been on the ability of the antifreeze formulation in the concentrate to avoid undue gelling. Yet, it is state-of-the-art practice for antifreeze manufacturers to recommend that antifreeze concentrates be diluted with water to provide a 50 volume percent working antifreeze (viz.--the antifreeze concentrate being diluted with an equal volume of water).
As discussed in commonly assigned application, Ser. No. 855,380, it has been found that many silicone stabilizers that are satisfactory in providing stability in an antifreeze concentrate appear to destabilize the silicate in the water-diluted or working antifreeze, causing a portion of the silicate corrosion inhibitor to form an insoluble species in solution. At the very least, this will result in a loss of corrosion inhibition efficiency which could well be substantial. This is considered to be a rather surprising discovery in light of the extensive prior efforts in this field.
This destabilization, upon dilution to provide a working antifreeze, thus presents a serious obstacle which must be overcome. Stability of the antifreeze composition in concentrate form provides no assurance that the concentrate, upon dilution with water to yield the working antifreeze, will retain the desired corrosion protection intended with a silicate antifreeze formulation. In addition to the potential loss of corrosion protection, destabilization may well result in silicate precipitation, causing blockage of the radiator tubes in an automobile cooling system.
Still further, it would be highly desirable to provide an antifreeze that is capable of retaining adequate stability, yet which allows greater latitude in formulation. For example, an antifreeze with a higher RA (i.e.--reserve alkalinity) is often desirable. This also allows maintenance of the desired working pH for a longer period of time. Yet, such a high RA composition generally tends to lessen the desired silicate stability. Similarly, it is often desirable to incorporate supplemental corrosion inhibitors, many of which are ionic in nature, yet the presence of ionic supplemental corrosion inhibitors likewise tends to exacerbate the silicate stability problem.
Still further, U.S. Pat. No. 4,367,154 discloses that the shelf life or gelation resistance of single phase glycol or glycol ester compositions containing alkali metal silicates can be improved by the addition to the glycol compositions one of the following groups of silanes: EQU (RO).sub.3 m(R').sub.m Si--R.sup.2 --O--P(O)(OR.sup.3)(R.sup.4), EQU [(RO).sub.3 m (R').sub.m Si--R.sup.2 --O--].sub.2 P(O)(R.sup.4), or EQU mixtures of I and II,
wherein m is 0-2
R, R.sup.3, R.sup.4 are alkyl groups of 1-4 carbons, PA1 R' is an alkyl group of 1-4 carbons, phenyl, and aralkyl group of 7-10 carbons, and PA1 R.sup.2 is an alkylene group of 1-4 carbons.
Typically, the methyl derivative of these materials which is produced as a precursor has the following structure: EQU (MeO).sub.3 Si(CH.sub.2).sub.3 --OP(O)(OMe)(Me)
This methyl ester precursor of these phosphorous modified silanes, however, is not soluble in ethylene glycol unless it has been partially saponified with an alkali metal salt such as sodium hydroxide. U.S. Pat. No. 4,370,255 is concerned with the saponified products. Such products are prepared by taking the silylalkylester of phosphorus, treating with the appropriate alkali metal hydroxide such as sodium hydroxide and then refluxing for several hours to saponify the phosphonate silane precursor.
However, it has been found that saponification of such phosphorous modified silanes can result in problems. Thus, when the degree of phosphonate ester group saponification which is desired is carried out, such saponification can result in the silane ester to phosphorous group being hydrolyzed, producing a silane species which is much less effective as a silicate stabilizer. This lack of hydrolytic stability is highly undesirable. Moreover, the degree of saponification that will actually be achieved using the same saponification conditions can vary, perhaps significantly, as can the by-products. This set of circumstances is, of course, less than desirable for a highly reliable and reproducible commercial process.
It would be desirable to be able to provide hydrolytically stable phosphonate silanes capable of imparting adequate stabilization in the antifreeze concentrate and also generate effective silicate species in the aqueous antifreeze.